Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or cooking spray.
In a large mixing bowl, combine the granulated sugar, flour, cocoa powder, baking powder, and salt. Whisk until well blended.
Stir in the milk, melted butter, and vanilla extract until the mixture is smooth and combined.
Pour the batter into the prepared baking dish, spreading it evenly.
In a separate bowl, mix the brown sugar and hot water together until the sugar is dissolved. Carefully pour this mixture over the batter in the baking dish. Do not stir.
If using, sprinkle the chopped nuts over the top.
Bake in the preheated oven for 30-35 minutes, or until the top is set and a toothpick inserted in the center comes out mostly clean.
Allow the cobbler to cool for about 10 minutes before serving. Serve warm with a scoop of vanilla ice cream if desired.